Hunt for the Higgs kicking into high gear

"In the next few years, if the Higgs lives in the expected mass range of 114 to 185 GeV, we will most probably see evidence of its existence," said Dmitri Denisov, co-spokesman of DZero and a Fermilab staff scientist, Sunday at the American Association for the Advancement of Science conference in Chicago.

If the Tevatron experiments collect data through 2011, their chance of finding evidence for the Higgs particle is larger than 30 percent for most of the Higgs mass region of interest, 114 to 185 GeV/c2. A Higgs mass of 170 GeV/c2 already is excluded by the analysis of Tevatron data last summer (see plot below).

The Higgs explains how particles acquire mass, and is the last, missing piece of the Standard Model, which explains matter and how it acts. Essentially, the Higgs is the last piece of a puzzle describing everything around the world, including ourselves, Denisov said.

While the Large Hadron Collider in Europe is the new, bigger kid on the high-energy physics' block, Tevatron researchers said their smaller experiments bring a finely-tuned machine and a head start to the game.

It might take the LHC until 2011 or 2012 to reach the sensitivity needed to find the Higgs, said Jim Virdee, spokesman of CMS and a professor at Imperial College of London.

Measurement constraints, including some from the Tevatron experiments, have narrowed in on the region considered the most likely home of a light-mass Higgs, the 114 to 185 GeV mass region.

LHC's so-called sweet-spot sits in the mass range of 155 to 170 GeV.

"The Standard Model Higgs (if it exists) is being produced now at the Tevatron--we have enough energy," said CDF co-spokesman Jacob Konigsberg, a University of Florida professor, who has worked with CDF since its beginning in the 1980s. "There is just not enough of them."

But that is quickly changing.

The Higgs rarely appears in particle collisions and when it does it hides amongst many other particles that mimic its traits. By increasing the number of particle collisions, scientists increase their probability of honing in on this tricky prey.

"We are setting records every week pretty much," Konigsberg said. "Every year this program gets better and better."

The more than a decade-old Tevatron accelerator complex operates more than 300 times better than it was initially designed to do. In the past five years at the Tevatron, the luminosity–-the number of collisions per second–-has increased sixfold. In a recent six-week period alone, overall luminosity improved 10 percent, generating more than a dozen luminosity records, sometimes multiple records in one week.

That increase in the production of rare particles and number of collisions has kept the Tevatron pumping out discoveries and physicists lining up to work on CDF and DZero.

The collaborations have seen an increase in the last year in the number of post docs and graduate students clamoring to join the discovery race.

In 2008, CDF and DZero work resulted in more than 100 papers submitted to ICHEP08. Particularly noteworthy analysis in 2008 were: improvement in the measurement of the top quark mass, new Higgs exclusion limits, observation of ZZ events, and the discovery of new b-quark bound states.

"The discoveries show we understand our experiments, we are doing very well and we are ready to take the next step and find the Higgs," Denisov said.

The Tevatron experiments are expected to have an easier time picking out the Higgs if it has low-mass, while the larger, higher-energy LHC will have a better reach for a high-mass Higgs.

It is expect that when the recent results are combined by this summer, the complementary approaches will enable the pair of experiments to exclude another region as a possible hiding spot for the Higgs, as the duo did with the 170 GeV range in the summer of 2008, or start to see an excess of events as first hints of the Higgs signal.

"We will continue to increase this exclusion region--or find the Higgs," Denisov said.

The Tevatron is expected to operate through 2009 and Fermilab has made a case to keep the discovery machine operating through 2011. With an additional two years of operation, the CDF and DZero experiments would have more than twice the data available today. That opens the door to compelling physics such as the first exploration of the Terascale, a better definition of top-quark mass and finding the Higgs boson.

DZero detector at Fermilab

The two detectors use slightly different technology to capture and analyze the same clues. Working in tandem, the detector teams can search mass and energy ranges more efficiently than they could alone. Each detector has its own strength. CDF has a larger magnet and dominates in tracking particle paths, which show electrical charges; while DZero has a larger calorimeter and dominates in measuring particle energy levels.

The detectors serve as a check and balance against each others' analyses, ensuring data anomalies don't get recorded as discoveries. Each research team records the types of particles, including quarks, electrons, and muons, that shoot off from the collisions and those particles' energy levels, decays, and electrical charges. Scientists test theories on particle interactions, measure particle properties, and look for discrepancies in calculations predicted by the Standard Model, the current blueprint for the universe. The results can indirectly reveal the existence of new physics and point the way to discoveries.

CDF assembly at Fermilab

Tag-team approach

The LHC and the Tevatron also take a competitive, but complementary approach to the search for the Higgs.

"We both work together on this, and we are both making advances," said Fermilab Director Pier Oddone. "It is a quest we undertake together, and we benefit only through the use of both of these machines."

At least in the first few years of LHC operation, the Tevatron and LHC will not look for the same Higgs production and decay mode, allowing the United States and Europe based experiments to complement each other.

Many LHC physicists also cut their teeth at the Tevatron experiments, providing data that helped point the LHC in the direction of the Higgs and continues to narrow the areas the LHC will have to spend time searching so it can instead focus on untapped energy ranges.

So while whether the Tevatron will find the Higgs first or not remains a question, its benefit to particle physics research world-wide goes unquestioned.

"We have this feeling we are right at the threshold of some great discovery," Oddone said. "It is like Columbus, if we get to the beach it will be very exciting. We won't be able to explore the full continent behind it, that is the job of the LHC."